The invention relates to new anisotropic compounds which carry at least one cross-polarizing substituent and offer reduced viscosities and increased clear points; the invention furthermore relates to liquid crystal mixtures (LC mixtures) containing these compounds as components.
As is known, nematic LC mixtures with negative .DELTA..epsilon. values are required for various types of electrooptical displays, for example for the so-called "guest/host displays" (GHD) and the homeotropic-nematic displays (HND), .DELTA..epsilon. being designated DC anisotropy or DCA and being defined as .DELTA..epsilon.=.epsilon..sub..parallel. -.epsilon..sub..perp., wherein .epsilon..sub..parallel. is the dielectric constant (DC) parallel to the longitudinal axis of the molecule and .epsilon..sub..perp. is the DC perpendicular to the molecular axis.
Furthermore, for certain types of operation of the displays, for example for multiplex operation, LC mixtures with an overall positive DCA but as small as possible a value of the ratio .DELTA..epsilon./.epsilon..sub..perp. are required, that is to say mixtures or compounds with a significant value for .epsilon..sub..perp..
The values of .epsilon..sub..parallel. and and .epsilon..sub..perp. are determined by the degree of polarization of the molecule in the direction parallel and, respectively, perpendicular to the longitudinal axis of the molecule; for example, a highly polarizing substituent, such as the cyano group or a halogen atom, as an end group--also designated a longitudinally polarizing substituent below--increases the value of .epsilon..sub..parallel., while such a substituent in the "lateral" position, that is to say more or less at right angles to the longitudinal direction of the molecule, increases the value of .epsilon..sub..perp. ; these lateral substituents are here designated "cross-polarizing" substituents.
However, the LC mixtures and their components must also fulfil a number of other characteristics, including, in particular, a high photochemical, general chemical, heat and electrochemical stability and as low as possible a viscosity and low optical anisotropy (low tendency towards double refraction) and clear points which are as high as possible.
According to the prior art, negative DCA values or higher .epsilon..sub..perp. contributions are generally achieved by providing an aromatic cyclic radical, usually a benzene ring, with at least one cross-polarizing substituent.
The known compounds with negative DCA thus have, as the characteristic element, cores having the structure ##STR2## in which X' is the cross-polarizing group, for example cyano or halogen, n is 1 or 2 and the arrows approximately correspond to the longitudinal axis of the molecule. Such compounds are described, for example, in German Pat. Nos. A-2,240,864, 2,613,293, 2,835,662, 2,836,086, 2,853,728 and 2,937,770 and European Pat. No. A-0,023,728.
It is furthermore known, from European Patent Application No. 79,200,259.4, that at least one cross-polarizing substituent can be combined with a longitudinally polarizing substituent to obtain anisotropic compounds with positive DCA and small values of the ratio .DELTA..epsilon./.epsilon..perp..
The known compounds have the common factor of widening of the molecule as a result of the cross-polarizing substituent or substituents, this being a disadvantage because it leads to a reduction in the clear point and frequently gives rise to a relatively high viscosity; since the lateral polarizing groups can in practice be introduced only into aromatic radicals, the necessity of the presence of an aromatic radical represents a further limitation.